1. Field of the Invention
The present invention relates to an image forming apparatus such as a duplicator or printer using a recording technique of an electrophotography system or electrostatic recording system, and more particularly to an image forming apparatus having a speed-changeable image bearing body.
2. Related Background Art
A laser printer using a transferring electrophotography process is taken as an example for the sake of convenience.
The above printer rotates an electrophotography photosensitive body (hereinafter, referred to as “photosensitive drum”), which is generally configured as a rotating drum, at a predetermined circumferential speed (process speed), charges a surface of the rotating photosensitive drum by a charging means to predetermined polarity and electric potential, and scans the charged surface to be exposed with a laser light. The laser light is modulated corresponding to a time-series, electric, digital, pixel signal of a purposed image information output from a laser scanner unit, so as to form an electrostatic latent image corresponding to the purposed image information on the surface of the rotating photosensitive drum. This electrostatic latent image is developed into a toner image by a developing means, and this toner image is transferred by a transferring means on a recording material (hereinafter, referred to as “recording paper”) fed from a feeding unit to the transferring unit of the rotating photosensitive drum at predetermined timing. And, the recording paper on which the toner image (unfixed image) is transferred is introduced to a fixing means so as to fix the toner image on the recording paper as a permanent, fixed image to be output as an image forming material.
Conventionally, a roller heating method as shown in FIG. 9 is generally used for the fixing means (fixing unit or fixing machine). Reference numeral 20 denotes a rotation fixing roller acting as a heating member, in which a halogen heater 21 heats from inside of the roller having a heatproof releasing layer such as PFA formed on a surface of a core metal such as aluminum in a hollow cylindrical shape, and the roller is maintained to a predetermined temperature by regulating ON/OFF operation of the heater 21 with a control circuit not shown based on a temperature detected by a temperature detecting element 22. By introducing and carrying a recording material P on which an unfixed toner image T is laid on a fixing nip part N, which is a compressing nip part with a pressing roller as a pressing member compressed to the fixing roller 20, the toner image T is melted by heat of the fixing roller 20 so to be heat-fixed (stuck) on the recording paper surface
On the other hand, as for the fixing device using the roller heating manner as described above, there is proposed a fixing device using a film heating manner (a heat-fixing device) characterized in that a power consumption is low in operation and a first print time, which is a time required from receiving a print signal till discharging a first piece of paper, is short, and disclosed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878 and so on. A schematic sectional view of the fixing device using the film heating manner is shown in FIG. 10.
This fixing device includes a heating member 30 having a heating body 32, a temperature detecting element 33 for detecting temperature of the heating body 32 and a heating body support 35 for supporting the heating body 32, and a pressing (pressuring) roller 36 interposed between the heating body 32 of the heating member 30 and a heat-resisting film 34, and introduces and carries a recording material P on which an unfixed toner image T is carried between the film and the pressing roller 36 of the fixing nip N acting as a compressing nip unit between the heating member 30 and the pressing roller 36 so that the toner image T is melted by the heat of the heating body 32 interposed by the film 34 so to be heat-fixed (stuck) to the recording material surface.
By using this film heating manner, it is possible to efficiently endow heat to the recording material P and fix the toner T with low power consumption because the thermal capacity of the whole fixing apparatus is low and the recording material P is faced with the heating body 32 only through the heat-resisting film 34. Furthermore, it is possible to operate the fixing apparatus rapid owing to the lower thermal capacity, and it is also possible to restrain power consumption and increase of inner temperature during not printing because it does not need to maintain residual heat as like the fixing apparatus using the roller heating manner.
In the above fixing apparatus using the film heating manner, a power source is commonly installed to the pressing roller 36 acting as a pressing member to drive the pressing roller 36 to rotate so that the heat-resisting film 34 acting as a moving body of the heating member 30 is driven, and the recording paper P introduced to the fixing nip part N is provided with carrying force from the rotation force of the pressing roller 36 so as to interposably carry the fixing nip part N together with the film 34. (The pressing roller driving method, disclosed in Japanese Patent Application Laid-Open No. 4-44075 to 44083, 204980 to 204984 and so on).
In case of the above pressing roller driving manner, because a carrying speed of the recording paper P in the fixing nip part N is dominated by a diameter of the pressing roller 36, there is caused a change of speed in that, compared with a carrying speed for a diameter of the pressing roller when the pressing roller 36 is not heated at the beginning of the fixing operation (an initial diameter), the carrying speed at the fixing nip part N is increased after performing the fixing operation continuously because the pressing roller 36 is heated by the heating member 30 and therefore thermally expanded so that the outside diameter is increased than the initial diameter.
For that reason, after executing the recording operation continuously, the recording paper carrying speed at the fixing nip part N of the fixing apparatus becomes higher than the recording paper carrying speed (process speed) at the transferring unit (image forming unit), thus creating a difference in speed between them. If the recording paper P is carried through the transferring unit at a predetermined process speed and passes the transferring unit while receiving the toner image as it is being transferred and a leading edge of the recording paper reaches the fixing nip part N of the fixing apparatus and is then interposed at the fixing nip part, in the case that the recording paper is ranged over the transferring unit and the fixing nip part N, this recording paper is carried at a recording paper carrying speed of the fixing nip, which is greater than the recording paper carrying speed of the transferring unit, or the predetermined process speed.
Therefore, at the point that the leading edge of the recording paper reaches the fixing nip part N of the fixing apparatus, because the recording paper is carried while being drawn at the recording paper carrying speed of the fixing nip part N of the fixing apparatus, which is higher than the recording paper carrying speed of the transferring unit, namely the predetermined process speed, the toner image (recording image) not transferred to the recording paper is stretched in a recording paper carrying direction, thus elongating the image. In some cases, a rear end of the image is deviated and lost from a rear end of the recording paper due to the image elongation.
To solve this problem, there is proposed an image forming apparatus to prevent the image elongation by changing the rotational speed of the photosensitive drum based on the successive recording paper number or the successive recording time, as disclosed in Japanese Patent Application Laid-Open No. 9-319282 (U.S. Pat. No. 5,819,149).